USING GEOCHEMISTRY AND TRACERS TO DIAGNOSE RUNOFF GENERATION IN THE MOUNTAINOUS SEASONAL TROPICS (Invited Presentation)

Water resources in the seasonal humid tropics are under increasing pressures from population growth, land surface alterations, and the looming threat of climate change. An important outstanding research questions is: What are the effects of land use on the quantity and quality of surface waters? The answer to this question has important implications for land use management to promote the abundance of clean water. Over the past eight years, we have continuously observed runoff in the 414 km² Upper Rio Chagres basin in the Panama Canal Watershed. We have also performed campaign-based water quality sampling with collaborators from The Ohio State University and North Carolina State University over this period. The Upper Rio Chagres is 98% covered by old growth forest and is perhaps the largest watershed of its' size with over 80 years of continuous flow measurements owing to the presence of the Panama Canal. Beginning in 2010, we established a network of water level sensors in the adjacent 313 km² Rio Pacora watershed, which is approximately 70% deforested and used for subsistance agriculture, primarily grazing of livestock. Additionally, we implemented a campaign-based water quality sampling campaign in that basin. Our process level explorations of runoff generation in these two catchments focuses on land use as it affects soil bioturbation, develops compaction layers, and alters rates of groundwater recharge. This presentation presents results comparing the hydrological and geochemical response of these paired basins. We also present results of electrical resistivity tomography tests using a salinity contrast and tracers to better understand the effects of land-use as it affect layering. We conclude with ongoing research activities to better understand these effects.